As an industrial machine such as a hydraulic shovel or a crane, remote-controlled machine or machine for automatic operation is increasingly introduced. Such industrial machine is equipped with a stereo camera or a laser scanner as a three-dimensional measuring sensor. In many cases, such three-dimensional measurement sensor conducts photographing in a front direction from a driver's seat, and it is not possible to monitor obstacles over the entire circumference of the industrial machine.
On the other hand, to monitor the obstacles surrounding the industrial machine, a surrounding monitoring camera including plural cameras is increasingly introduced. In the surrounding monitoring camera, since plural combined monocular cameras are used, calculation of a distance from a detected obstacle is limited. Generally, for distance measurement with a monocular camera, there are limitations, for example, the size of the object should be already known, or distance information on the photographed ground should be already known. To eliminate these limitations, sensors to measure a distance from an object or ground in a photographing range of the monocular camera may be provided. In a distance measuring method disclosed in [Patent Literature 1], in addition to the surrounding monitoring camera, a distance image sensor based on TOF (Time of Flight) method or the like is provided for distance measurement.